Method for stretch forming tapered blanks



Dec. 17, 1957 H. w. HEIN L n-AL ,8

MEETHOD FOR STRETCH FORMiNG TAPERED BLANKS Filed June 21, 1956 3 Sheets-She et 1 By 8M Dec. 17, 1957 METHOD FOR STRETCH FORMING TAPERED BLANKS Filed June 21, 1956 III" I JG [7 3 Sheets-Sheet. 2

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METHOD FOR STRETCH FORMING TAPERED BLANKS Henry W. Hein, Chardon, and Frank J. Phillips, Cleve land, Qhio, assignors to The Cyril Bath Company, Solon, Ohio, a corporation of Ohio Application June 21, 1956, Serial No. 592,855

3 Claims. (Cl. 15340) This invention relates to a method and apparatus for forming tapered blanks of sheet metal, rod stock, and the like, and particularly for forming blanks of sheet metal stock of which the taper results from a decrease in the width of the stock progressively from one end toward the other.

In the stretch forming of metal stock about a side face die considerable difficulty is encountered in obtaining the proper tension on the stock at the instantaneous point of contact with the die if the cross section of the stock is different at different locations along the length the stock. Quite often the difference in cross section at the contact point is due to differences in the thickness of the metal but more generally to the decrease, progressively from one end of the stock to the other, in width of a sheet of metal of constant thickness.

Often, for airplane bodies, sheet metal skin elements required must be curved flatwise of the sheet about an axis extending lengthwise of the sheet and also are curved flatwise of the sheet about an axis extending crosswise of the sheet. These elements must also be tapered in width endwise.

In the stretch forming of such sheets considerable difficulty is encountered because the total tension required for forming the small cross-section end of the sheet is much less than that required for stressing the larger crosssection end of the stock to the same unit tension.

In accordance with the present method, the stock is stretch formed progressively from one end to the other in increments of such length that there need not be too great a variation in the unit tension at either end of the increment. In general, this is accomplished by anchoring the small end of the stock, tensioning the stock endwise to the maximum unit tension permissable for the cross section of the small end, then, while maintaining the tension, progressively laying the stock on the side face of the die over a short increment of the length of the stock; next binding the stock against the side face of the die near that end of the formed increment which is nearer to the large end of the stock, and, while thus holding the stock fixedly against the die, increasing the unit tension to the maxi mum permitted by the cross section at the instantaneous line of contact of the stock and die, and stretching it toward the larger end while progressively laying another increment onto the die face; then relieving the pressure and binding the stock against the die near that end of the new increment which is nearer the larger end, and continuing the sequence by increments progressively from the small end toward the larger end of the stock.

Again, the result may be obtained by gripping the stock at the large end and holding it firmly against the die near the large end and thereupon laying it on the die while holding it under tension applied at the smaller end plus tension applied by the frictional drag of a wiping shoe moving toward the smaller end from the larger end. The tension at the start is that required for stretch forming the large end. During the wiping of the stock toward the small end, the total tensioning force employed is g nja W I l gradually decreased by decreasing the pressure at which the wiping shoe is pressed against the stock so that the total tensioning force due to tension applied to the small end and the frictional drag is gradually decreased in conformance with that required for the cross section of the particular increment at the instantaneous point of Wipe forming.

The matter in which the above method may be practiced effectively will become apparent from the following description of a suitable apparatus therefor, wherein reference is made to the drawings, in which:

Fig. 1 is a top plan view of an apparatus for practicing the present method;

Fig. 2 is a vertical sectional view of the apparatus illustrated in Fig. 1. and is taken on the line 22 thereof;

Fig. 3 is an enlarged fragmentary perspective view illustrating the cooperation of the wiping means and die with the sheet or stock being formed; and,

Fig. 4 is a perspective view of the resultant formed tapered piece.

The apparatus for the practice of the method may be of the general type shown and described in the copending application of Stanley M. Dolney and Henry W. Hein, Serial No. 368,876, filed July 20, 1953, and entitled, Method and Apparatus for Forming Elongated Metal Stock.

Referring to Fig. l, the apparatus comprises a turntable l which may be rotatably driven in opposite directions, selectively, by means of a suitable rack 2 and pinion 3. The rack 2, in turn, may be driven by a suitable piston and cylinder assemblage 4 which comprises a cylinder 5 and a piston having a rod 6.

Mounted on the turntable is a tensioning piston and cylinder assemblage 7 which includes a cylinder 8 and a piston having a rod 9. Connected to the piston rod 9 is a carrier it) supported by guide rods 11 on a suitable support 12 for guided movement forwardly and rearwardly of the cylinder 8.

Mounted on the carrier 10 is a gripping head 13 having a plurality of jaws 14- which are adapted to engage one end of a length of metal stock 3 for exerting tension thereon. The head 14- is arranged so that the gripped end of the stock may be curved cross-wise somewhat near to the curvature the formed stock is to have adjacent this particular gripped end. The turntable 1 and its operating mechanism are mounted on a rigid frame 15 which supports, in spaced relation to the turntable, a main stretch forming piston and cylinder assemblage 16. This assemblage includes a cylinder 17 and a piston having a rod is which rod, in turn, is connected to a pedestal 1) of a slide 26 which is arran ed to move parallel to the axis of the cylinder 17. The slide 2ft carries a gripping head 21 having a plurality of jaws 22 so that it can grip the larger end of a length of stock and hold it approximately in conformity with the shape the finished stock is to have, near the gripped portion, in the finished article.

The assemblage i5 is mounted on a carriage 23 which is adjustable to different positions lengthwise of the frame l5 toward and away from the table. The assemblage 16 i mounted on the carriage 23 by means of an upright pivot so that it may swing about the pivotal axis and thus align itself and the stock in tangential relation to the side face of the forming die from the instantaneous point of contact of the stock with the die toward the large end of the stock.

A forming die D, having the desired curvature about the upright axis of the table and about a horizontal axis, is mounted on the table 1 for rotation therewith.

In addition to the frame 15, a lateral frame 25 is provided, the frame 25 being connected to the frame 15 in fixed relation thereto. The lateral frame 25 is provided with guideways 26 which extend lengthwise of the frame 15. Mounted on the guideways is a carriage 27 which is reciprocable along the guideways 26 by means of a piston and cylinder assemblage 28. Mounted on the carriage is a saddle 29. The saddle is connected to the carriage by means of a pivot 30 so that it can rotate about an upright pivotal axis.

On the saddle is a piston and cylinder assemblage 31, including a cylinder 32 and a piston having a rod 33, the piston being arranged with its axis generally parallel to the plane of the top of the turntable 1 and extending generally toward and away from the turntable.

Mounted in a suitable guideway on the saddle 29 is a slide 34 which is guided for movement parallel to the axis of the piston and cylinder assemblage 31. On one end of the slide is an upright pedestal 35 on which is mounted a wiping and holding shoe 36. The pedestal 35 is connected to the rod 33 of the piston. Operatively interposed between the carriage 27 and the saddle 29 is a piston and cylinder assemblage 37 which is operable to oscillate the saddle 29 to dififerent operative positions about the axis of the pivot 30 and to lock the saddle in the preselected oscillated position about the axis of the pivot 36.

The holding and wiping shoe 36 is preferably such as described in the copending application of August Yurka, Jr., Serial No. 592,795, filed of even date herewith, and entitled, Wipe Forming and Holding Shoe.

The shoe 36 and the assemblage 31 are such that the pressure can be applied to strip stock, such as indicated at S, and hold it frictionally against the face of the die D with sufficient binding stress to withstand the maximum pull which is to be exerted by the gripping head 21 on the stock at the cross section determined by the in stantaneous line of tangency between the stock and die.

Thus, in operation, a length of stock S can be gripped, preferably at its small end by the gripping head 13, the assemblage 7 being locked hydraulically. Thereupon, pressure fluid is admitted to the assemblage 16 so as to place the stock under tension endwise. At the time of starting, this tension is determined by the maximum stress which can be withstood by the cross section of the stock at the small end or adjacent the gripping head 13. While this condition is maintained, the turntable is rotated counterclockwise in Fig. 1 so as to wrap on the side face of the die, an increment of the stock extending from the small end for a short distance part way along the length of the die toward the large end. After a few inches or a predetermined increment has thus been formed, depending upon the tension required and the degree of taper, the shoe 36 is released and relocated nearer the larger end of the stock, or wiped part way from the small end of the stock toward the large end over an increment which ends at the instantaneous line of tangency of the stock and die. If the shoe is wiped toward the large end, the tension applied by the assemblage 16 plus the tension due to frictional drag of the shoe 36 must not exceed, during Wiping, that permissible for the small end of the stock. If desired, the shoe 36 may be release-cl and moved an increment of stock length in which case no allowance need be made for frictional drag. In each instance, the shoe is applied firmly against the stock so as to bind it frictionally tightly against the face of the die at the instantaneous point of application of the shoe 36.

While the stock is thus held, the pressure on the assemblage 16 is increased to provide the maximum resultant tension allowable for the smallest cross section of the next increment, which is the end of the new increment nearest the shoe. The table is then operated to stretch form the next increment of the length of the stock onto the die. When this next short increment is laid onto the die, tension applied by the assemblage 16 is reduced, the shoe 36 wiped toward the larger end or is withdrawn and reapplied to the stock again near to the larger end, and with sufiicient force to frictionally lock the stock against the die at the larger end of said next increment, which end is at the instantaneous line of tangency of the stock and die. Thereupon the stock is again tensioned at a higher total pull so as to provide the adequate unit tension for the increased cross section of the latest increment of stock length. This operation is repeated progressively from the small end toward the large end of the stock by successive increments of length of the stock, the forming tension applied by the assemblage 16 being increased for each increment due to the increase in the cross section of the successive increments.

If the shoe is released between shifts from one increment to another, a minimum tension, not exceeding that required to form the smallest end of the stock, is maintained on the stock at all times during repositioning of the shoe 36.

Due to the swivelling of the assemblage 31 under the control of the assemblage 37, and to the movement of the assemblage 31 along the guideways 26 by means of the assemblage 28, the shoe 36 can be kept in fixed position on the stock during the short increments of rotation of the table.

By making the increments which are progressively stretch formed sufficiently small, the difference between the unit stress on the small end of each increment and that on the large end of the same increment, can be reduced to a negligible amount so that in effect the piece is stretched at all points along its length at substantially the same unit tension.

In some instances, a more continuous operation can be obtained by stretching from the large end toward the small end. In such an instance, the large end of the stock is gripped by the head 13 and the small end by the head 21. Tension is then applied to the small end of the stock equal to the maximum which the small end requires for formation. The shoe 36 is applied first adjacent the head 13 at the larger end of the stock and then, with the stretch forming tension applied by the assemblage 16 kept constant, the shoe 36 is applied with pressure which is progressively decreased as the shoe progresses from the large end of the stock adjacent the head 13 toward the small end of the stock adjacent the head 21. Thus the constant tension applied by the assemblage 16 plus the tension due to frictional drag produced by the shoe 36 is sutficient to stretch the large end of the stock. This frictional tensional stress due to the shoe 36 is gradually decreased as the smaller end of the stock gripped by the head 21 is approached.

In order to provide the desired pressure on the stock for binding it by the shoe 36, the shoe 36 is arranged to conform as near as may be to the cross section of the die face adjacent the instantaneous point of tangency. For such purposes the front or operating face of the shoe 36 should be made self-adjusting, all as more fully set forth in the above identified copending application.

It is apparent, therefore, that by the tensioning of successive increments of the length of stock progressively from one end toward the other, the stock can be stressed to substantially the optimum tension required for formation at each increment of cross section along its length whether the difference in cross section is due to the width of the stock or to its thickness, or to combinations thereof.

Having thus described our invention, we claim:

1. The method of stretch forming a strip of sheet metal, of which the cross sectional area is decrescent from one end toward the other end of the strip, against a side face die, of which the die face is curved both crosswise and transversely of the strip, and comprising anchoring one end of the strip in fixed position relative to the die, applying tension to the strip endwise of the strip from the other end, applying transverse pressure to the face of the strip so as to press the strip firmly against, and into conformance with, the die face at a plurality of locations endwise of the strip progressively from the anchored end toward the other end, varying the total tensioning force to which the strip is subjected in predetermined relation to the cross sectional area at said locations so that the total tension efiective on each increment of length of the strip between the instantaneous line of tangency of the strip and die and the small end of the strip is less than that efiective on each increment of length of the strip between said instantaneous line of tangency and the large end of the stock.

2. The method according to claim 1 characterized in that the small end of the strip is the one which is anchored, the required transverse pressure, to the degree necessary for binding the strip to the die, is applied intermittently at said increments, and the tension is at least reduced between applications of the required degree of transverse pressure and is reapplied in increased degree, successively, as the increments are nearer the large end.

3. The method of stretch forming according to claim 1 characterized in that the large end of the strip is the one which is anchored, the transverse pressure is applied continuously progressively from the large end to the small end, and the total tension created by the tension applied to the small end and the frictional drag due to application of the transverse force is progressively decreased as the instantaneous line of tangency of the stock and die approaches the small end.

References Cited in the file of this patent UNITED STATES PATENTS 2,411,801 Nielsen Nov. 26, 1946 2,412,731 Hoffman Dec. 17, 1946 2,514,830 Bath July 11, 1950 

